1. Field of the Invention
The present invention relates to a structure of a semiconductor laser device capable of operating with high output power and high reliability.
2. Description of the Prior Art
FIG. 1 is a view showing a sectional structure of a conventional semiconductor laser device having an inner stripe structure, disclosed, for example, in Japanese Patent Laid-Open Gazette 1727289/1982. In FIG. 1, a conventional semiconductor laser device comprises a p type GaAs semiconductor substrate 1 on which there are provided a p type AlGaAs semiconductor active layer 4 for lasing, a lower cladding layer 3 of p type AlGaAs and an upper cladding layer 5 of n type AlGaAs, these lower and upper cladding layers 3 and 5 being formed in contact with the lower surface and the upper surface of the p type AlGaAs active layer 4, respectively, and both having a larger forbidden band width (band gap) than that of the active layer 4. Between the p type GaAs semiconductor substrate 1 and the p type AlGaAs lower cladding layer 3, an n type GaAs current blocking layer 2 is formed. In addition, a V-shaped groove 7 for ensuring a stable laser addition in a fundamental transverse mode is formed to extend through the n type GaAs current blocking layer 2. On the n type AlGaAs upper cladding layer 5, a contact layer 6 of n type GaAs is formed. On other surface of the n type GaAs contact layer 6 and on other surface of the p type GaAs semiconductor substrate 1, metal electrodes 8a and 8b are formed, respectively. In the following, an operation of the above described conventional device will be described.
A voltage is applied between the metal electrodes 8a and 8b in the forward direction with respect to the pn junction formed in the interface between the active layer 4 and the upper cladding layer 5. As a result, a forward-flowing current narrowed by the V-shaped groove 7 enters the active layer 4 so that light is emitted. The light is guided through a waveguide originating from a difference of the refractive indices between the active layer 4 and the cladding layers 3 and 5 and an effective refractive index difference in the active layer 4 and the V-shaped groove 7 so that lasing is performed by a resonator formed by both cleaved facets (provided along the direction parallel to the surface of the drawing).
In such a conventional semiconductor laser device mainly comprised of AlGaAs, radiating a light in a short wavelength band, regions near the laser light emitting facets serve as regions for absorbing the laser light based on the surface levels. As a result, the maximum light output power is limited by a catastrophic optical damage (referred to hereinafter as COD) at the light emitting facets, which obstructs high-output operation.
For the purpose of removing such obstruction to high-output operation, a method of increasing the light emitting area to decrease optical density may be considered. Such method is adopted in the above described conventional semiconductor laser device in which the thickness of the active layer 4 is made thinner to enhance infiltration of light from the active layer 4 whereby the light emitting area is increased. However, in the conventional semiconductor laser device where the thickness of the active layer is made thinner, the amount of the laser light contributing to induced emission in the active layer is decreased by the infiltration of light. As a result, the lasing threshold current is increased and as a more serious problem, it is confirmed that the longevity of a semiconductor laser device conspicuously deteriorates if the thickness of the active layer is decreased to a certain value or less.
FIG. 2 is a graph showing a relation between the life characteristics of a conventional semiconductor laser device mainly comprised of Al.sub.x Ga.sub.1-x As and the thickness of an active layer, which is indicated in Journal of Applied Physics, Vol. 56, pp. 3088 (1984). In FIG. 2, the vertical axis represents a change .DELTA.Id of driving current in a period after 4 hours to 24 hours in the case of constant light output operation with the conditions of 50.degree. C. - 5 mW/facet. As the change .DELTA.Id increases, degradation becomes more conspicuous. The horizontal axis represents a lasing wavelength, which corresponds to the thickness of an active layer because the aluminum density (X value) is fixed in this case. If the lasing wavelength is shorter than 742 nm, deterioration in quality becomes conspicuous as can be seem from FIG. 2. The lasing wavelength 742 nm corresponds to the thickness of an active layer of 0.06 .mu.m. In other words, if the thickness of an active layer becomes smaller than 0.06 .mu.m, the life characteristics are rapidly deteriorated even in the case of low light output operation of 5 mW. Similar phenomena were recognized in laser devices of other structure.
FIG. 3 is a graph showing a relation between the thickness of an active layer in a channeled substrate planar laser device of an active layer curving type and the percentage of reliable laser devices in each wafer, which is shown in Kazimura et al., Proceedings of Autumn Meeting of Japan Applied Physics Society, 30a-B-8 p. 135 (1982). In FIG. 3, the vertical axis represents the percentage of reliable laser devices in each wafer assuming that the devices where the decreasing rate of the light output power in constant current operation for 24 hours is 10% or less with an initial light output power of 3 mW/facet at 50.degree. C. are regarded as reliable laser devices. The horizontal axis represents the thickness of an active layer. In addition, X.sub.ACT and X.sub.clad indicate X values of an active layer and a cladding layer of Al.sub.x Ga.sub.1-x As, respectively. As can be seen from FIG. 3, the percentage of reliable laser devices is remarkably lowered in a wafer where the thickness of an active layer is smaller than 0.04 .mu.m.
FIG. 4 is a graph showing a relation between the thickness of an active layer in a twin-ridge-substrate type laser device and a percentage of degraded laser devices, which is disclosed in Hamada et al., IECE Japan Technical Report, Optical Quantum Electronics Group reference OQE 83-59, P.1, 1983. In FIG. 4, the vertical axis represents a percentage of degraded laser devices in a period from the time when 50 hours have passed to the time when 100 hours have passed after the start of a reliability test, and the horizontal axis represents the thickness of an active layer. As can be seen from FIG. 4, if the thickness of an active layer is smaller than 0.05 .mu.m, the percentage of degradation tends to increase.
A relation between the thickness of an active layer and the life time in operation in a short wavelength laser device is disclosed in "Highly Reliable GaAlAs Visible-Light-Emitting MCSP Lasers", T. Kazimura et al., Japanese Journal of Applied Physics, Vol. 22 supplement 22-1, pp. 325 to 328, 1983. In this paper, it is verified that the life time in operation is extremely decreased if the thickness of the whole area of an active layer is made smaller than 0.04 .mu.m.
As described above, such a conventional method for obtaining high output power by making thinner the whole area of an active layer involves a problem that the life characteristics are extremely deteriorated if the thickness of an active layer is decreased to 0.04 to 0.06 .mu.m or less.